1. Field of the Invention
The present invention relates generally to a signal processing method and more particularly to a signal processing method in an electromagnetic test (to be referred to as ECT herebelow). The test measures a defect, a physical quantity, and a geometrical quantity on a surface of a conductive test object or in the proximity of the surface thereof by the use of an induced current (an eddy current) and a defect detector using the electromagnetic induction test.
2. Discussion of Background
The fundamental principle of the ECT is that a coil with an alternating current (with a frequency ranging from 100 Hz to several megaherz) flowing therethrough is placed in the neighborhood of a conductive test object. A current (eddy current) is induced therethrough and the disturbance of the induced current is detected as a change in the impedance of the coil or a change in the induced voltage, thereby detecting a defect, a physical quantity, and a geometric quantity of the test object. Incidentally, in addition to the defect, the physical quantity, and the geometric quantity to be detected, the phase detection output of the ECT also depends on such various factors or parameters at the same time as the lift-off (the distance between the coil and the test surface), the roughness of the test surface, and temperatures of the coil and the test object which effect the induced current. Consequently, for an ECT with a high precision, it is necessary to separate the various noise factors from the factors of the detection object.
FIGS. 9a-9b are explanatory diagrams illustrating one of the conventional examples of the signal processing method for separating and removing the noise factors in the ECT ("Eddy Current Defect Detecting Test B", NDI, '84, p. 114). This conventional example can be applied to a case where only a single noise factor is present, the alternating current (ac) to be applied to the coil has a single test frequency, and a coordinate rotation is effected as shown in FIG. 9a so that in a plane in which two orthogonal axes are formed with phase detection outputs X and Y obtained from the eddy current defect detector, the indication of the noise factor is parallel to one of these axes. FIG. 9b is a block diagram illustrating an apparatus for effecting the processing of the coordinate rotation in which reference numeral 1 indicates a coordinate rotating unit. With the provision of the coordinate rotation processing, the signals in a direction orthogonal to the axis parallel to the indication of the noise factor become free from the effect of the noise, which makes it possible to detect with a high precision a signal corresponding to the factor or parameter of the detection object to be measured.
However, the indication of the noise factor does not actually lead to a sharp image of indication as shown in FIG. 9a. Namely, there exists a fluctuation of the values in this direction. Consequently, the noise factor cannot be removed to a sufficient degree in the prior art example.
To overcome this difficulty, there has been disclosed a method (the Japanese patent laid-open No. 59-163559) in which a predetermined range .alpha..degree. is set for the noise phase angle as shown in FIG. 10. Any indication in this range is regarded as a noise indication and is therefore masked so as to remove the effect of the noise. In this diagram, reference symbols p and q indicate a noise signal and a defect signal, respectively.
On the other hand, in a case including a plurality of noise factors, there has been conventionally used a method in which a plurality of (multiple) test frequencies are used corresponding to the noise factors. FIG. 11 shows an example of such a method in which reference numerals 2a-2f are coordinate rotating units. The rotation angles thereof are appropriately set beforehand so that the indication of each noise factor matches with the coordinate axis. In a multidimensional space finally comprising orthogonal axes of the phase detection outputs associated with the respective test frequency, a direction orthogonal to the indication of each noise factor is selected, thereby detecting a signal corresponding to the factor or parameter of the detection object to be measured without being effected from the noise ("Eddy Current Defecting Test B", NDI, '85, p. 118)., In FIG. 11, reference symbol d indicates a defect and reference symbols s and w denote noise factors.
In addition, the following method has also been disclosed as a simplified version of the method described above. In this method, two test frequencies are used. First, the vector plane is subjected to a coordinate rotation so that the component of the noise factor related with a first frequency of these frequencies and that related with a second frequency thereof match with one of the orthogonal axes of a vector plane of each frequency. Thereafter, a component orthogonal to the noise factor component related to the first frequency and a component orthogonal to the defect factor component related to the second frequency are extracted, and then these components are set to be orthogonal to each other in a new vector plane, thereby discriminating the defect factor and the noise factor based on the trace of signals on the plane (Japanese patent laid-open No. 60-146149).
As described above, in any conventional example described above, a phase angle or a phase angle range of the noise indication is beforehand examined and then the signal processing is accomplished by use of the coordinate rotation angle set according to the examined data.
However, the phase angle is actually subject to fluctuation because of variations in the lift-off, the coil temperature, the temperature of the test object, and electric characteristics (such as the electric conductivity and the magnetic permeability). Consequently, even if a phase angle or a phase angle range is beforehand established as in the case of the conventional example, the coordinate rotation angle is actually changed. Even if the coordinate rotation angle is slightly varied, the precision of the detection and measurement of the ECT is greatly deteriorated.